The continuous thermofixation installations developed by applicant usually include an enclosure known as a steaming enclosure or thermofixation chamber, and two chambers known as cold chambers arranged on both sides of the thermofixation chamber with each being separated from the thermofixation chamber by an intermediate chamber. The steaming enclosure is generally filled with saturated pressurized steam, that is circulated across the belt and the threads carried thereon, with the temperature of the steaming chamber being maintained at a predetermined temperature, greater than 100.degree. C. The upstream and downstream cold chambers are preferably connected between them by an appropriate conduit, to permit an equalization of the pressures, and are fed with pressurized air by a blower which permits the creation of a slight overpressure with respect to the average pressure prevailing inside the steaming enclosure. The presence of the cold chambers with overpressure and intermediate chambers furnished with lock chambers are provided with the aim of avoiding great losses of steam generated within the steaming enclosure, and thereby reduce energy consumption.
In the majority of known installations in service today, for example according to French Patent FR-A-2, 453, 927, the upstream cold chamber is relatively short and in principle does not have any particular function in the framework of the treatment process of the thread, of not to prevent, as much as possible, escape of steam, by virtue of the surrounding overpressure.
The average length of the upstream cold chamber is currently on the order of 0.5 meters. The downstream chamber currently has a length of 2 meters given that it plays a supplementary role with respect to that of the upstream chamber, i.e. that of ensuring a precooling of the threads at the outlet of the steaming enclosure. The evolution of the temperature in such a known installation is illustrated by FIG. 7 which will be described in detail below.
It has been observed that after a momentary stop of the conveyor belt carrying the threads, the material situated during this stop in a zone near the outlet of the upstream cold chamber or in the intermediate chamber, and that which is at this moment upstream from this zone, obtained different tinctorial affinities. In practice, this translates, after treatment, into lengths of thread in which the colorant has a concentration other than that on the rest of the thread, which results in the appearance of lines or bands which are lighter or darker through a cloth or a piece of fabric. This consequence constitutes, of course, a disadvantage since it results in a product having defects.
Analysis of the phenomenon has permitted the observation that during the stop of the conveyor belt of threads to be thermofixed, the heat present inside the steaming enclosure is diffused through the lock chamber delimiting the intermediate chamber and spreads progressively into the cold chamber. The result is a preheating of the threads situated in the outlet zone of the cold chamber or in the corresponding intermediate chamber, this preheating having the consequence of a modification in the structure of the fibers having been used for the fabrication of the thread, with this modification of structure leading to a modification in the tinctorial affinity. It has been deduced that the sudden passage of the threads from the upstream cold chamber towards the steaming enclosure caused for the thread a thermal shock which is more significant when the conveyor belt is in motion than when it remained stopped for some time, and that the difference between the thermal shocks sustained by the thread under normal transport conditions and after the stop are at the origin of these modifications of tinctorial affinity leading to defects in the products manufactured by means of these threads.